1. Field of the Invention
The present invention concerns a hydridable material for the negative electrode in a nickel-metal hydride storage battery. It also concerns the use of this material in the negative electrode of a nickel-metal hydride storage battery.
2. Description of the Prior Art
Sealed nickel-metal hydride storage batteries are alkaline storage batteries with aqueous electrolytes; the reactant constituted by the hydrogen is stored in the mass of the hydridable alloy which has the ability to absorb hydrogen in large quantities. This alloy must be able to store and give up the hydrogen depending on whether the storage battery is being charged or discharged, at a rate which is adequate for normal operating conditions. It must also have a high electrochemical capacity, resist corrosion in the potassium hydroxide and be non-toxic.
A great deal of work has been carried out on intermetallic AB.sub.2 type compounds known as "Lave's phases". These phases are classified into three crystalline structures, cubic C15 (MgCu.sub.2 type), hexagonal C14 (MgZn.sub.2), and hexagonal C36 (MgNi.sub.2). The latter symmetry is little used. These phases can be obtained from ZrCr.sub.2 and ZrV.sub.2 systems and from TiMn.sub.x, HfMn.sub.x and ZrMn.sub.x systems where x is between 1.5 and 2.5. These materials, however, are difficult to use industrially due to acute oxidizability in air, corrosion in the electrolyte and passivation during use in an electrode.
European patent application EP-A-0 587 503 describes a family of hydridable materials comprising a major phase with a C14 structure and general formula: EQU (Zr.sub.1-a A.sub.a)(Ni.sub.1-(b+c+d+e) Mn.sub.b Al.sub.c Co.sub.d M.sub.e).sub.t
where ##EQU2## where A represents at least one element selected from Ti, Y, Ce, Ca and Mg and M is selected from Cr and Si.
While these materials have a relatively high capacity, the majority of them are difficult to charge and discharge quickly with satisfactory performance. In addition, multiphase materials are vulnerable to corrosion due to the phenomenon of localized micro-cells, which reduces the performance on cycling.
The present invention particularly concerns a hydridable electrode material which can be charged and discharged quickly in satisfactory fashion.